Automatic return-to-center deflection circuit

ABSTRACT

An automatic return-to-center deflection circuit develops a deflection signal that linearly increases from a minimum value to a maximum value and deflects a scanning beam in a display device from a first extreme position to a second extreme position. When the deflection signal reaches its predetermined maximum value, the scanning beam is automatically retraced to the center or midpoint position between the first and second extreme positions. The scanning beam is then retraced in a second step to the first extreme position when a new scanning cycle is initiated.

United States Patent Inventor Appl. No. Filed Patented AssigneeAUTOMATIC RETURN-TO-CENTER DEFLECTION CIRCUIT Assistant Examiner--BrianL. Ribando Attorney-H. Christofl'ersen ABSTRACT: An automaticreturn-to-center deflection circuit develops a deflection signal thatlinearly increases from a minimum value to a maximum value and deflectsa scanning beam in a display device from a first extreme position to asecond extreme position. When the'ldeflection signal reaches 7 Clams 3Drawmg Figs its predetermined maximum value, the scanning beam is auto-U.S. Cl 315/19, matically retraced to the center or midpoint positionbetween 315/24 the first and second extreme positions. The scanning beamis Int. Cl H01j 29/70 then retraced in a second step to the firstextreme position Field of Search 315/19, 24 when a new scanning cycle isinitiated.

2' 38 Y DEFLECTN 7 POWER SUPPLY HO 14 l- AMPLIFIER y y i i* CIRCUIT 36 I1 CHARACTER 2 GENERATOR DEFL. 2O 24 AND COIL CONTROL x ,T: 30 ClRCUlTRYT PATENTEDHAR 9197:

' sum 2 0F 2 INVliN'I'UR. GUS F/LSO/V CARROLL ATTORNEY BACKGROUND OF THEINVENTION Display devices are utilized to provide a visual display ofdata being written into or read from a computer. A display terminalusually includes both a display device and a character generator alongwith the associated control circuitry therefor. in a dispiay terminal,there is usually one character generator for each display device andfrequently the character generator is integrally united with the displaydevice. However, it is frequently desirable to make the charactergenerator a separate entity from the display device and then have aplurality of display devices operated from a single character generator.

In one particular system, a monoscope tube is utilized as the charactergenerator and a plurality of display devices are operated therefrom. Amonoscope generates one character at a time. Therefore, in order todisplay different characters on a plurality of difi'erent displaydevices, it is necessary to couple the monoscope to the differentdisplay devices in succession. Furthermore, to avoid complexity,monoscopes typically generate each separate character in the same amountof time. Consequently, there is a maximum number of characters that canbe generated by a monoscope in any practical system and the maximumnumber of characters must be divided proportionately for each displaydevice, when a plurality of display devices are operated simultaneously.For example, when four display devices are operated simultaneously eachreceives one quarter of the maximum number of characters available, etc.For operating efficiency, the display devices are paired with each otherso that when one display device displays characters, the scanning beamin the other display device is retraced. rleretofore, the scanning beamwas retraced to its initial extrerne deflection position to remain in astandby condition until a new line of characters was to be displayed.Since most energy in a scanning beam is dissipated while standing by insuch an extreme deflection position, a lot of energy is wasted.

SUMMARY OF THE INVENTION A return-to-center deflection circuit for adisplay device embodying the invention includes deflection means forproviding a deflection signal that linearly increases from a minimumvalue to a maximum value to deflect a scanning beam in said device froma first extreme position to a second extreme position. First retracemeans are included in the device for retracing the scanning beam in onestep from the said second extreme position to a center rest positionlocated substantially midway between the first and second extremepositions after the deflection signal reaches its maximum value. Secondretrace means are included in the device for retracing the scanning beamin a second step from said center position to the first extreme positionat the beginning of a new scanning cycle. linearly BRIEF DESCRIPTION OFTHE DRAWZNGS FIG. 1 is an overall block diagram of a video data terminalincluding a detailed schematic circuit diagram of a deflection circuitembodying the invention;

FIG. 2 is a graphical representation of a series of signal waveformsthat are helpful in understanding the operation of the deflectioncircuit in FIG. 1; and

FIG. 3 is a detailed schematic circuit diagram of another deflectioncircuit embodying the invention.

DETAILED DESCRIPTION Referring to H6. i there is shown a displayterminal it that includes a deflection circuit 12 embodying theinvention. A character generator 14 which may, for example, include amonoscope tube, as well as the associated control circuitry therefor,generates video signals that represent characters,

which are then applied to a display device to. The display device inmay, for example, comprise a cathode ray tube, that includes a face 38for displaying the: characters. A scanning beam 24), which emanates froma cathode 22 in the tube to, is directed to impinge against the phosphoron the inside of the face l8 so as to produce a light spot 24 thereon.The scanning spot 24 is deflected by X direction (i.e. horizontal) and Ydirection (i.e. vertical) deflection coils 2b and 28. The X and Ydeflection coils 2d and 28 are coarse deflection coils. Characters areformed on the face M by additionally superimposing onto the scanningbeam Ztl a tickler" deflection signal by means of a tickler coil (notshown) which deflects the scanning spot 24 up and down in. a rapidmotion. The character generator I4 turns the scanning beam 20 on and of?at predetermined points during this tickler deflection to create eachcharacter by a plurality of vertical slices. As a consequence,characters 30, such as the capital letter T, are formed on the face E8of the display device lid. The synchronizing signals for deflection inthe X direction are obtained from the character generator l4 and appliedto the X direction deflection circuit 12. The output deflection in thedeflection circuit 112 are AC (i.e. alternating current) coupled via acapacitor 32 to a variable gain amplifier 34 and thence to the Xdeflection coil 26. Similarly, the: character generator 14 also suppliesthe synchronizingsignals to a Y direction deflection circuit 112'. Thedeflection circuit 12' is similar to the deflection circuit l2 exceptfor the fact that since this is a Y direction deflection circuit, thecircuit 12 is operated with different time constants because there willbe many horizontal scan cycles for each vertical scan cycle. Thevertical deflection circuit 12' is AC coupled via capacitor 36 to avariable gain amplifier 38 and thence to the Y direction deflection coil28. A power supply, so is coupled to supply the power to the variouscircuits in the video data terminal 116.

Referring now in more detail to the deflection circuit 12, this circuitincludes deflection means 39 for developing a deflection signal thatlinearly increases from a minimum value to a maximum value to cause thescanning spot 24 to deflect from an extreme left position to an extremeright position on the face 18 of the tube 16. The deflection means 39includes a reactive circuit element 42 that is coupled to be charged bya constant current source 44. The reactive circuit element 42 isillustrated in FIG. 1 as a capacitor which is charged by a currentsource. In accordance with the principle of duality, the reactivecircuit element may also comprise an inductor that is coupled to becharged by a constant voltage source. The current source 44 is shownvariable to denote that different currents may be supplied to thereactive circuit element 42 when different pluralities of displaydevices are coupled to the character generator i4.

First retrace means 45 is included in the deflection circuit l2 toretrace the scanning beam 20 from an extreme righthand position on theface T8 of the display device l6 back to the center of the face. Thefirst retrace means includes a pair of complementary transistors 46 andd8 of opposite conductivity types. The first transistor 46, which is ofPNl'P-type conductivity, includes an input emitter electrode d7 that iscoupled to the constant current source M and an output collectorelectrode $6) that is coupled through a resistor 52 to a terminal 54supplying a negative potential V,. The collector electrode 56) is alsocoupled through a resistor 56 to the base electrode 58 of the secondtransistor 38. The second transistor db is of NPN-type conductivity andincludes an emitter electrode bit that is coupled to the terminal 54 aswell as a collector electrode 62 that is coupled to one terminal 54 aswell as a collector electrode 62 that is coupled to one terminal 64 of avoltage divider 6b. The voltage divider ss includes a pair of resistorses and 7b that are coupled between the divider terminal 64 and aterminal 72 that supplies the positive potential +V The midpoint 71. ofthe voltage divider as, which is the junction of the resistors til; and7b, is coupled to the base electrode 74 of the first transistor as.

Second retrace means 75 is included in the deflection circuit 12 toretrace the scanning beam 20 from the center of the *ace 18 to the leftside thereof. The second retrace means 75 includes a transistor 80 ofthe NPN-type conductivity. The transistor 80 includes an emitterelectrode 82 coupled to the negative potential terminal 54 and acollector electrode 84 coupled through a resistor 86 to the junction ofthe capacitor 42 and the emitter electrode of the transistor 46. Thus,the second retrace means 75 is directly shunted across the capacitor 42.Synchronizing signals that are derived from the character generator 14are applied through a resistor 88 to the second retrace means 75 at thebeginning of each scanning cycle.

OPERATION In describing the operation of the deflection circuit 12 ofFIG. 1, reference will be made to the waveforms 90 and 92. The waveform92 in FIG. 2 is the synchronizing signal applied from the charactergenerator to initiate the scanning cycle. The waveform 90 comprises theX direction deflection signal that is developed across the reactivecircuit element 42 and applied via the capacitor 32 and the amplifier 34to the X direction deflection coil 36.

it is assumed that operation begins at the time t shown in FIG. 2. Atthis time t all of the transistors 46, 48 and 80 in the deflectioncircuit 12 are off or nonconducting. Consequently, the current derivedfrom constant current source 44 is applied to the reactive circuitelement 42. The reactive circuit element 42 therefore develops alinearly increasing voltage that increases from a minimum value ofamplitude A to a maximum value of amplitude A This linearly increasingportion of the waveform 90 causes the scanning beam to deflect 20 todeflect from an extreme left side position of the display face 18 to anextreme right side position thereof. Of course, superimposed on thisdeflection is the tickler deflection signal which produces individualslices or scans of characters in a plurality of vertical deflections.

When the maximum value A is reached, 12 of transistor 46 becomes forwardbiased and is rendered conductive. This is because the potential +V isselected such that the maximum value A exceeds the potential +V by theemitter-base voltage drop of the transistor 46. The transistor 46 inconducting diverts a portion of the current from the source 44 away fromthe reactive circuit element 42 and The base 58 of the transistor 48 istherefore raised from the negative potential level V, and thebase-emitter junction becomes forward biased. The transistor 48therefore also turns on and the current flow through the transistor 48produces a voltage drop in the voltage divider 66. This voltage dropreduces the potential at the base of the transistor 46 which in turndiverts a larger portion of the current from the source 44 away from thereactive circuit element 42. The interconnections of the transistors 46and 48 provide a regenerative feedback path that rapidly increases thecurrent flow through both transistors and partially discharges thereactive circuit element 42. Such a discharge is shown by the portion ofthe waveform 92 between the times t, and t At the time t the transistors48 saturates and the voltage across this transistor 48 becomessubstantially fixed at a relatively small magnitude. The base 74 of thetransistor 46 is therefore held fixed at a potential determined by thevoltage divider 66. The resistance value of the voltage divider areselected to hold the transistor '46 conducting in a desired manner. Thevoltage across the reactive circuit element 42 is therefore fixed at theamplitude A which is substantially midway between A and A as shown inH0. 2. The scanning beam 20 is therefore positioned at the center of thetube 16 midway between the left and right extreme positions ofdeflection on the display face 18. At this point, no power is beingutilized in the X deflection coil 26. Any tendency for the voltageacross the reactive circuit element 42 to change is counteracted by thetransistors 46 and 48. The transistors 46 and 48 therefore exhibit avoltage regulation between the times t;, and t Such a voltage regulationefiectiv'ely converts the constant current source 44 into a constantvoltage source, which maintains the scanning beam fixed at the centerrest position of the display device 16.

At the time t,, a synchronizing pulse 100 (FIG. 2) derived from thecharacter generator 14 is applied to the base electrode of thetransistor in the circuit 12. The synchronizing pulse 100 is effectiveto deflect the scanning beam 20 from the center rest position to theextreme left position of the display face 18 so that a new scanningcycle is initiated. The synchronizing pulse 100 forward biases and turnson the transistor 80 to effectively convert the transistor 80 into aclosed switch across the reactive circuit element 42. The transitor 80therefore discharges the reactive circuit element 42. Such a dischargeis shown by the portion of the waveform between the times t and t Duringthis discharge, the drop in voltage across the capacitor 42 reversebiases the emitterbase junction of the transistor 46 and turns off thistransistor. The turnoff of the transistor 46 also turns off thetransistor 48. Consequently, the current from the constant currentsource 44 is diverted through the transistor 80. The width of thesynchronizing pulse is selectedsuch that the pulse ends when thereactive circuit element discharges to the minimum value of voltage AWhen the synchronizing pulse 100 terminates, the transistor 80 isrendered nonconductive. At this point, all the transistors 46, 48 and 80are nonconductive and a new scanning cycle has begun.

It is to be noted that the return-to-center of the scanning beams 20during retrace reduces the power dissipation in the X deflection coil26. Such a reduction permits a smaller power supply 40 to be utilized inthe terminal 10 when a plurality of devices are operated simultaneously.

It is also to be noted that the variable feature of the current source44 permits changes in the time it takes for the capacitor 42 to chargefrom the value A to the value A Increasing the current from the source44 causes the capacitor 42 to charge more rapidly to the magnitude A asshown by the dotdashed deflection waveform 90a 'in FIG. 2. The number ofcharacters that can be displayed on the display device 16 is thereforereduced since the scanning beam 26 sweeps from the extreme left sideposition to the extreme right side position more rapidly than is thecase with the deflection waveform 90. Reducing the current from thesource 44 causes the capacitor 42 to charge more slowly to the value Aas shown by the dotted waveform 90b in FIG. 2. More characters cantherefore be displayedon the display device 16. Consequently more orless display devices can be operated simultaneously from the samecharacter reference generator 14. Of course, the repetition rate of theretrace pulse 100 in the waveform 92 also varies to correspond to thechanges in the current generator 44. Regardless of the time it takes thecapacitor 42 to charge to the magnitude A the deflection circuit 12still returns the scanning beam 20 to the center standby position whenthe magnitude A is reached.

This increase or decrease in the number of characters displayed issubstantially independent of the gain exhibited by the amplifier 34.l-ieretofore, scanline time and character number changes wereaccomplished by changing the gain of an amplifier, (e.g. amplifier 34)which in turn affected the centering of the scanning beam 20. Areadjustment of the centering control in turn affected the gain. Furtheradjustments and readjustments were therefore necessary. Such adjustmentsand readjustments are obviated by utilizing the deflection circuit 12.

It is also to be noted that the deflection circuit 12 renders thedisplay terminal 10 operator proof." If the operator forgets to adjustthe synchronizing pulses 108 when more or less display devices are addedto or subtracted from the data terminal 10, no harm is done to theterminal 161. This is because no resultant displacement of the scanningbeam 20 can occur when the deflection waveforms 90 are AC coupledthrough the capacitor 32 to the display device 16.

In FIG. 3 there is shown another embodiment of the deflection circuit12. In FIG. 3 parts corresponding to those in FIG. 2 are given the samereference numerals but the numerals are primed. The values and types ofcomponents are also shown in FIG. 3. It is to be noted that the constantcurrent source 44 comprises a transistor 110 that is coupled through apotentiometer 112 and a resistor 114 to a positive potential supply +VThe potentiometer 112 has a plurality of taps labeled 2, 4i, 6 and 8 foradjusting the deflection limits when 2, 4, 6, and 3 display devices areoperated in one terminal. The base of the transistor 110 is also coupledthrough a resistor 116 to the positive potential +V A pair of oppositelypoled diodes 118 and 120 are coupled between the voltage divider 66 andthe collector of the transistor 110. The diode 120 prevents current fromthe voltage supply +V= from flowing into the capacitor 42' and the diode118 maintains the correct potentials on the transistor 46'. The outputof the reactive circuit element 42 is coupled through a two-stageemitter-follower amplifier circuit comprising the transistors 122 and124.

Thus, in accordance with the invention, a return-to-center deflectioncircuit is provided which enables a video data terminal to operate aplurality of display devices from one character generator without largepower dissipation. The deflection circuit also provides the ability tochoose a wide variety of scan times with essentially constant powerdissipation. Furthermore, extreme flexibility is provided by the simplecontrols in the circuit which maintains minimal power dissipationregardless of system timing charges or operator adjustment errors.

Iclaim:

1. A deflection circuit for a display device that includes a scanningbeam, comprising in combination:

deflection means for developing a deflection signal that linearlyincreases from a minimum value to a maximum value to deflect saidscanning beam from a first extreme position on said display device to asecond extreme posi tion;

first retrace means for retracing scanning beam in a first step fromsaid second extreme position to a center position substantially midwaybetween the first and second extreme positions when said deflectionsignal reaches said maximum Value; and

second retrace means for retracing the scanning beam in a second stepfrom said center position to said'first extreme 6 l v 4 position at thebeginning of a scanning cycle. 2. The combination in accordance withclaim 1 wherein said deflection means includes:

a reactive circuit element for developing said deflection signal that isapplied to deflectsaid. scanning beam; and

means for applying substantially constant valued signal to saidreactivecircuit element to charge said reactive element to develop asubstantially linearly increasing deflection signal.

3. The combination in accordance with claim 2 that further includes:

means for altering the magnitudes of said substantially constant valuedsignal to change the time it takes to deflect said scanning beam fromsaid first extreme position to said second extreme position.

4. The combination in accordance with claim 2 wherein said first retracemeans includes a regulation circuit that is coupled to the reactivecircuit element to be operated when said reactive circuit elementcharges to said maximum value so as to discharge said reactive circuitelement to a value substantially midway between said minimum and maximumvalues to maintain said reactive circuit element at said midway value.

5. The combination is accordance with claim 4 wherein said secondretrace means includes a switching element coupled across said reactivecircuit element to be operated to close at the beginning of a scanningcycle to discharge said reactive circuit element to said first minimumvalue so as to retrace said scanning beam to said first extreme osition.

6. The combination in accordance WII clarm 4 wherein said regulationcircuit includes first and second complementary transistors with eachincluding base, emitter and collector electrodes.

7. The combination is accordance with claim 6 wherein said regulationcircuit further includes:

means for coupling the collector emitter of said first transistor to thebase electrode of said second transistor; a voltage divider coupledbetween a point of fixed potential and the collector electrode of saidsecond transistor; means for coupling the base electrode of the firsttransitor to point of said voltage divider; and means for coupling saidreactive circuit element from the emitter electrode of said firsttransistor to the emitter electrode of said second complementarytransistor.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,569,773 Dated March 9, 1971 Inventorflx) GUS Filson Carroll It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

Column 1, line 56, cancel "linearly"; line 56, t heading "BRIEFDESCRIPTION OF THE DRAWINGS" Should be on a separate line, Column 2,line 22, after "deflection" (sec occurrence) insert ---signalsdeve1oped---; lines 68 and 6 cancel "54 as well as a collector electrode62 that is coupled to one terminal" Column 3, line 33, cancel "todeflect"; line 40, "12 of" should read --the---; line 46, after "and"insert ---this current produces a voltage drop the resistor 52.---; line65, "vlaue" should read ---value Column 4, line 31, "beams" should read---beam---; line 50 cancel "reference" Column 5, line 38, after"retracing" insert ---said---; line 42, "Value" should read ---value--Column 6, line 23, "is" should read ---in---; line 40, aft "to" insert---a---; line 40, "of" should read ---on---.

Signed and sealed this 25th day of January 1972.

(SEAL) Attest:

EDWARD M.F'LEICHER ,JR. RQBEBT GOTTS ALK att ti Offic r Commissioner ofPatents

1. A deflection circuit for a display device that includes a scanningbeam, comprising in combination: deflection means for developing adeflection signal that linearly increases from a minimum value to amaximum value to deflect said scanning beam from a first extremeposition on said display device to a second extreme position; firstretrace means for retracing scanning beam in a first step from saidsecond extreme position to a center position substantially midwaybetween the first and second extreme positions when saId deflectionsignal reaches said maximum Value; and second retrace means forretracing the scanning beam in a second step from said center positionto said first extreme position at the beginning of a scanning cycle. 2.The combination in accordance with claim 1 wherein said deflection meansincludes: a reactive circuit element for developing said deflectionsignal that is applied to deflect said scanning beam; and means forapplying substantially constant valued signal to said reactive circuitelement to charge said reactive element to develop a substantiallylinearly increasing deflection signal.
 3. The combination in accordancewith claim 2 that further includes: means for altering the magnitudes ofsaid substantially constant valued signal to change the time it takes todeflect said scanning beam from said first extreme position to saidsecond extreme position.
 4. The combination in accordance with claim 2wherein said first retrace means includes a regulation circuit that iscoupled to the reactive circuit element to be operated when saidreactive circuit element charges to said maximum value so as todischarge said reactive circuit element to a value substantially midwaybetween said minimum and maximum values to maintain said reactivecircuit element at said midway value.
 5. The combination is accordancewith claim 4 wherein said second retrace means includes a switchingelement coupled across said reactive circuit element to be operated toclose at the beginning of a scanning cycle to discharge said reactivecircuit element to said first minimum value so as to retrace saidscanning beam to said first extreme position.
 6. The combination inaccordance with claim 4 wherein said regulation circuit includes firstand second complementary transistors with each including base, emitterand collector electrodes.
 7. The combination is accordance with claim 6wherein said regulation circuit further includes: means for coupling thecollector emitter of said first transistor to the base electrode of saidsecond transistor; a voltage divider coupled between a point of fixedpotential and the collector electrode of said second transistor; meansfor coupling the base electrode of the first transitor to point of saidvoltage divider; and means for coupling said reactive circuit elementfrom the emitter electrode of said first transistor to the emitterelectrode of said second complementary transistor.